Acute lung injury (All) results from loss of the alveolar-capillary barrier with high-permeability pulmonary edema and alveolar flooding. Disruption of endothelial cell (EC) barrier function secondary to paracellular gap formation and cytoskeleton changes is critical in ALI, which causes significant morbidity and mortality. HMGB1, a late mediator of sepsis, plays a role in the evolution of ALI and is unique in sepsis as its levels rise later than those of other proinflammatory cytokines, offering the potential to interfere with its effects in a clinically relevant time frame. The mechanism of HMGB1 's role in ALI is unknown. The PI, a nascent physician-scientist, has generated compelling preliminary data which support the hypothesis that HMGB1 signaling to the actin cytoskeleton results in EC barrier disruption and contributes to ALI pathobiology. The Specific Aims of this K08 are designed: SA#1. To define the receptor involved in HMGB1 -mediated EC barrier disruption. The PI will utilize human lung EC and siRNA and dominant negative forms of three known HMGB1 receptors to define the receptor(s) involved. SA#2. To define the cell signaling molecules involved in signal transduction after HMGB1-receptor binding. Molecular and pharmacologic approaches will be utilized to define the signaling mechanisms involved in HMGB1-mediated EC barrier disruption. SA#3. To define cross-talk between HMGB1 and heat shock protein (Hsp) 27, which stabilizes the actin cytoskeleton in EC to maintain cell shape, using mutant forms of Hsp27 to study the role of Hsp27 phosphorylation in HMGB1 - mediated EC barrier disruption. In vivo studies will utilize relevant knockout mice and in vivo siRNA. These tools along with excellent mentorship will allow the PI to achieve the goals of this application. These will elucidate the mechanism by which HMGB1 disrupts endothelial cells and contributes to ALI, providing a molecular target for potential therapeutic intervention in this devastating disorder. Relevance: Acute lung injury (ALI) results from many insults, one of the most common being overwhelming systemic infection (sepsis). One mediator of sepsis, HMGB1, is unique because its levels rise late enough for physicians to interfere with its actions, and is also involved in the development of ALI. This proposal aims to define the mechanism by which HMGB1 disrupts lung endothelial cell barrier function and contributes to ALI, opening the door for potential therapeutic intervention in the future.